High strength steel plate superior in stretch flange formability and fatigue characteristics

ABSTRACT

The present invention provides high strength hot rolled steel plate superior in stretch flange formability and fatigue characteristics comprising steel plate containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%, having a balance of iron and unavoidable impurities and having a number ratio of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 μm or more and a long axis/short axis of 5 or more of 20% or less.

TECHNICAL FIELD

The present invention relates to high strength hot rolled steel platesuperior in stretch flange formability and fatigue characteristicssuitable as a material for members of an automobile chassis.

BACKGROUND ART

From the viewpoint of improvement of automobile safety and improvementof the fuel economy leading in turn to environmental protection, thedemands for increasing the strength and reducing the weight of the hotrolled steel plate used for automobiles have been growing stronger.Among auto parts, in particular, the weight of frames, arms, etc. called“chassis parts” accounts for a high ratio of the weight of the vehicleas a whole, so the materials used for such locations are being madehigher in strength and smaller in thickness to enable lighter weight.Further, the materials used for such chassis parts are required to havehigh fatigue characteristics from the viewpoint of durability withrespect to the vibration during driving.

However, along with the higher strength and fatigue resistance, the holeexpandability tends to drop in the same way as the ductility. When usinghigh strength steel plate for the complicatedly shaped chassis partsetc. of automobiles, this hole expandability becomes an important matterfor study.

For this reason, several types of steel plates designed to achieve boththe mechanical strength characteristics and the fatigue characteristicsand hole expandability (workability) have been proposed. For example,Japanese Patent Publication (A) No. 11-199973 proposes steel platecomprised of composite structure steel plate of a ferrite phase and amartensite phase in which fine Cu is precipitated or a solid solution isdispersed (in general called “DP steel plate”). In the technologydisclosed in this Japanese Patent Publication (A) No. 11-199973, it wasfound that the solid solution Cu or CU precipitates comprised of Cualone and having a particle size of 2 nm or less are extremely effectivefor improving the fatigue characteristics and do not impair theworkability either. The ratios of compositions of the variousingredients were limited based on this.

It is known that such DP steel plate is superior in the balance ofstrength and ductility and in the fatigue characteristics, but thestretch flange formability, evaluated by a hole expansion test, remainsinferior. One of the reasons is believed to be that DP steel plate is acomposite of a soft ferrite phase and a hard martensite phase, so at thetime of hole expansion, the boundary parts of the two phases cannot keepup with the deformation and easily become starting points for breakage.

As opposed to this, high strength hot rolled steel plate satisfying notonly the fatigue characteristics, but also the tough demands for stretchflange formability for materials of recent wheels or chassis parts hasbeen proposed (for example, see Japanese Patent Publication (A) No.2001-200331). In the technology disclosed in Japanese Patent Publication(A) No. 2001-200331, the C is made as low as possible to make the mainphase a bainite structure and introduce a solution strengthened orprecipitation strengthened ferrite structure in a suitable volume ratio,reduce the difference in hardness of the ferrite and bainite, andfurther avoid formation of coarse carbides.

DISCLOSURE OF THE INVENTION

High strength hot rolled steel plate having a steel plate structure ofmainly a bainite phase and suppressing the formation of coarse carbidessuch as disclosed in said Japanese Patent Publication (A) No.2001-200331 does indeed exhibit a superior stretch flange formability,but cannot necessarily be said to be superior in fatigue characteristicscompared with DP steel plate containing Cu. Further, with justsuppressing the formation of coarse carbides, it is not possible toprevent the occurrence of cracks at the time of extreme hole expansion.According to the research of the inventors, the cause is the presence ofstretched sulfide-based inclusions mainly comprised of MnS in the steelplate. Upon repeated deformation, internal defects form near thestretched coarse MnS-based inclusions present at the surface layer orits vicinity and propagate as cracks to thereby cause deterioration ofthe fatigue characteristics. Again, stretched coarse MnS-basedinclusions easily become starting points of cracking at the time of holeexpansion. For this reason, it is preferable not to allow the MnS-basedinclusions in the steel to stretch as much as possible but to make themfinely spherical.

However, Mn is an element effectively contributing to the increase instrength of a material along with C and Si, but with high strength steelplate, to secure strength, the general practice has been to set theconcentration of Mn high. Furthermore, if not performing the overlappingtreatment of desulfurization in the secondary refining process, an Sconcentration of 50 ppm or more ends up being included. For this reason,a cast slab usually contains MnS. If the cast slab is hot rolled andcold rolled, the MnS easily deforms, so becomes stretched MnS-basedinclusions. These become causes lowering the fatigue characteristics andstretch flange formability (hole expandability). However, no example hasbeen found proposing hot rolled steel plate superior in stretch flangeformability and fatigue characteristics from the viewpoint of control ofthe precipitation and deformation of MnS.

Therefore, the present invention was proposed in consideration of theabove points and has as its object the provision of high strength steelplate superior in stretch flange formability and fatigue characteristicsimproving the stretch flange formability and the fatigue characteristicsby causing the precipitation of fine MnS in the cast slab and makingthis disperse as fine spherical inclusions not deformed and not easilybecoming starting points of cracking in the steel plate at the time ofrolling.

To solve the problems described above, the inventors engaged in in-depthstudies on the method of making fine MnS precipitate in cast slabs andmaking this disperse as fine spherical inclusions not deformed and noteasily becoming starting points of cracking at the time of rolling andto clarify the additive elements not causing deterioration of thefatigue characteristics. As a result, they learned that MnS precipitateson the fine, hard Ce oxides, La oxides, cerium oxysulfides, andlanthanum oxysulfides formed due to deoxidation due to addition of Ceand La, the thus precipitated MnS is resistant to deformation at thetime or rolling as well, so the amount of stretched coarse MnS in thesteel plate is remarkably reduced and, at the time of repeateddeformation or at the time of hole expansion, these MnS-based inclusionsdo not easily become starting points of cracking or routes for crackpropagation and that this leads to an improvement in the fatigueresistance etc.

The high strength steel plate superior in stretch flange formability andfatigue characteristics according to the present invention has as itsgist the following:

(1) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having a number ratio of stretchedinclusions present in the steel plate having a circle equivalentdiameter of 1 μm or more and a long axis/short axis of 5 or more of 20%or less.

(2) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having inclusions in the steelplate comprised of an oxide or oxysulfide of one or both of Ce or La onwhich MnS is precipitated in a number ratio of 10% or more.

(3) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having a volume number ratio ofstretched inclusions present in the steel plate having a circleequivalent diameter of 1 μm or more and a long axis/short axis of 5 ormore of 1.0×10⁴/mm³ or less.

(4) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having a volume number density ofinclusions in the steel plate comprised of an oxide or oxysulfide of oneor both of Ce or La on which MnS is precipitated in a volume numberdensity of 1.0×10³/mm³ or more.

(5) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having an average circle equivalentdiameter of stretched inclusions present in the steel plate having acircle equivalent diameter of 1 μm or more and a long axis/short axis of5 or more of 10 μm or less.

(6) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, having inclusions present in the steelplate comprising an oxide or oxysulfide of one or both of Ce or La onwhich MnS is precipitated, and having the inclusions include, in averagecomposition, a total of one or both of Ce or La in 0.5 to 50 mass %.

(7) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having a (Ce+La)/S ratio of 0.1 to70.

(8) A high strength steel plate superior in stretch flange formabilityand fatigue characteristics as set forth in any one of (1) to (7)characterized by comprising steel plate containing, by mass %, one ormore of any of Nb: 0.01 to 0.10%, V: 0.01 to 0.05%, Cr: 0.01 to 0.6%,Mo: 0.01 to 0.4%, and B: 0.0003 to 0.03% and having a balance of ironand unavoidable impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the relationship of Ce+La (%) and S (%).

BEST MODE FOR CARRYING OUT THE INVENTION

Below, as the best mode for carrying out the present invention, highstrength steel plate superior in stretch flange formability and fatiguecharacteristics will be studied in detail. Below, the “mass %” in thecomposition will be simply described as “%”.

First, the experiments leading to the completion of the presentinvention will be explained.

The inventors deoxidized molten steel containing C, 0.07%, Si: 0.2%, Mn:1.2%, P: 0.01% or less, S: 0.005%, and N: 0.003% and having a balance ofFe using various elements and produced steel ingots. They hot rolled theobtained steel ingots to obtain 3 mm hot rolled steel plate. They thenused the thus produced hot rolled steel plate for hole expansion testsand fatigue tests and investigated the number density, form, and averagecomposition of the inclusions in the steel plate.

As a result, they learned that steel plate not deoxidized much at all byAl, but given Si, then given at least Ce and La for deoxidation was themost superior in stretch flange formability and fatigue characteristics.The reason is that MnS precipitates on fine, hard Ce oxides, La oxides,cerium oxysulfides, and lanthanum oxysulfides formed due to deoxidationdue to addition of Ce and La, the precipitated MnS is resistant todeformation at the time of rolling as well, and therefore the stretchedcoarse MnS remarkably decreases in the steel plate. As a result, theseMnS-based inclusions do not easily become starting points of cracking orroutes of crack propagation at the time of repeated deformation or atthe time of hole expansion. This leads to improvement of the fatigueresistance etc. as explained above.

Note that the reason why the Ce oxides, La oxides, cerium oxysulfides,and lanthanum oxysulfides become finer is that the SiO₂-based inclusionsfirst formed by Si deoxidation are reduced and broken up by the lateradded Ce and La to form fine Ce oxides, La oxides, cerium oxysulfides,and lanthanum oxysulfides and, furthermore, the interfacial energybetween the formed Ce oxides, La oxides, cerium oxysulfides, andlanthanum oxysulfides themselves and the molten steel is low, soclustering after formation is also suppressed.

Based on the findings obtained from these experimental studies, asexplained below, the inventors studied the conditions for the chemicalingredients of steel plate and completed the present invention.

Below, the reasons for limiting the chemical ingredients in the presentinvention will be explained.

C: 0.03 to 0.20%

C is the most basic element for controlling the quenchability andstrength of steel. It increases the hardness and depth of the quenchedhardened layer and effectively contributes to the improvement of thefatigue strength. That is, this C is an essential element for securingthe strength of steel plate. To obtain high strength steel plate, atleast 0.03% is necessary. However, if this C is excessively included,the C is fixed by the formation of Ti carbides like in the past or evenif using cooling conditions, a cementite phase ends up being formed.This cementite phase causes work hardening of the steel plate and is notpreferable for improvement of the stretch flange formabilitycharacteristics. For this reason, in the present invention, from theviewpoint of improving the workability, the concentration of C is made0.20% or less.

Si: 0.08 to 1.5%

Si becomes an important deoxidizing element in molten steel to which Alor Ti are not added as much as possible like in the present invention,so is extremely important in the present invention. Further, Si has thefunction of increasing the nucleation sites of austenite at the time ofquenching heating and suppressing the grain growth of the austenite andof making the grain size of the quenched hardened layer finer. This Sisuppresses carbide formation and suppresses the drop in grain boundarystrength due to carbides. Furthermore, this Si is effective against theformation of a bainite structure as well and plays an important role interms of securing the strength of the material as a whole. To lower theconcentration of solute oxygen in the molten steel and cause theformation of SiO₂-based inclusions once (to reduce the SiO₂-basedinclusions by the later added Ce and La and thereby make the inclusionsfiner), it is necessary to add Si in 0.08% or more. For this reason, inthe present invention, the lower limit of Si was made 0.08%. As opposedto this, if the Si concentration is too high, the concentration of SiO₂in the inclusions becomes higher and large inclusions become easier toform or the toughness and ductility become extremely poor and thesurface decarburization and surface flaws increase, so the fatiguecharacteristics conversely deteriorate. In addition to this, ifexcessively adding Si, the weldability and the ductility aredetrimentally affected. For this reason, in the present invention, theupper limit of the Si was made 1.5%.

Mn: 1.0 to 3.0%

Mn is an element useful for deoxidization in the steelmaking stage.Along with C and Si, it is an element effective for raising the strengthof the steel plate. To obtain this effect, it is necessary to includethis Mn in 1.0% or more. However, if Mn is included in an amount over3.0%, the ductility drops due to the segregation of Mn and solutionstrengthening. Further, the weldability and matrix toughness alsodeteriorate, so the upper limit of Mn is made 3.0%.

P: 0.05% or less

P is effective in the point of acting as a substitution type solutionstrengthening element smaller than Fe atoms, but segregates at the grainboundaries of the austenite and causes a drop in the grain boundarystrength, so causes a drop in the torsional fatigue strength.Deterioration of the workability is a concern, so the amount is made0.05% or less. Further, if not necessary for solution strengthening, Pdoes not have to be added. The lower limit value of P therefore includes0%.

S: 0.0005% or more

S segregates as an impurity. S forms coarse stretched inclusions of MnSand causes deterioration in the stretch flange formability, so as low aconcentration as possible is desirable. In the past, to secure stretchflange formability, the concentration of S had to be made an ultralowone of less than 0.0005%. However, in the present invention, fine MnS ismade to precipitate on the hard Ce oxides, La oxides, ceriumoxysulfides, and lanthanum oxysulfides to make deformation at the timeof rolling difficult and prevent stretching of the inclusions, so theupper limit value of the concentration of S is not particularly defined.

Further, to reduce the S concentration to a level equal to the past ofless than 0.0005%, it is necessary to considerably strengthen thedesulfurization in the secondary refining. The cost of thedesulfurization for achieving this concentration becomes too high andthe effect of controlling the shape of the MnS becomes difficult toobtain, so the lower limit value of the S concentration is made 0.0005%.

N: 0.0005 to 0.01%

N is an element which is unavoidably mixed in the steel since nitrogenin the air is taken in during the melting process. N forms nitridestogether with Al, Ti, etc. to promote the increased fineness of thematrix structure. However, if overly adding this N, even with a fineamount of Al or a fine amount of Ti, coarse precipitates are formed andthe stretch flange formability is degraded. For this reason, in thepresent invention, the upper limit of the concentration of N was made0.01%. On the other hand, to make the concentration of N less than0.0005%, the cost becomes high, so 0.0005% is made the lower limit.

Acid soluble Al: 0.01% or less

With acid soluble Al, the oxides easily cluster and become coarse, sothis is preferably suppressed as much as possible to preventdeterioration of the stretch flange formability and the fatiguecharacteristics. However, use as a preliminary deoxidizing material upto 0.01% is allowed. This is because if the acid soluble Alconcentration is over 0.01%, the Al₂O₃ content in the inclusions exceeds50% and the inclusions cluster. From the viewpoint of preventingclustering, the lower the acid soluble Al concentration the better. Thelower limit value includes 0%. Further, the “acid soluble Alconcentration” measures the concentration of Al dissolved in an acid, sois a method of analysis utilizing the fact that solute Al dissolves inacid while Al₂O₃ does not dissolve in acid. Here, the “acid” means, forexample, a mixed acid of a mixture of hydrochloric acid in 1 part,nitric acid in 1 part, and water in 2 parts (mass ratio). Using such anacid, it is possible to separate acid soluble Al and Al₂O₃ notdissolving in an acid and measure the acid soluble Al concentration.

Acid soluble Ti: less than 0.008%

With acid soluble Ti as well, the oxides easily cluster and becomecoarse. Further, this bonds with the N in the steel to form coarse TiNinclusions. Therefore, the acid soluble Ti is made less than 0.008%. Thelower limit value includes 0%. Further, the “acid soluble Ticoncentration” measures the concentration of Ti dissolved in an acid, sois a method of analysis utilizing the fact that solute Ti dissolved inacid, while Ti oxide does not dissolve in acid. Here, the “acid” means,for example, a mixed acid of a mixture of hydrochloric acid in 1 part,nitric acid in 1 part, and water in 2 parts (mass ratio). Using such anacid, it is possible to separate acid soluble Ti and Ti oxides notdissolving in an acid and measure the acid soluble Ti concentration.

Total of one or both of Ce or La: 0.0005 to 0.04%

Ce and La have the effect of reducing the SiO₂ produced by Sideoxidation and forming inclusions having Ce oxides (for example, Ce₂O₃,CeO₂), cerium oxysulfides (for example, Ce₂O₂S), La oxides (for example,La₂O₃, LaO₂), lanthanum oxysulfides (for example, La₂O₂S), Ce oxide-Laoxides, or cerium oxysulfide-lanthanum oxysulfides as main phases (50%or more as a rule of thumb) which easily become Mn precipitating sitesand are hard, fine, and resistant to deformation at the time of rolling.

Here, these inclusions sometimes also partially contain MnO, SiO₂, orAl₂O₃ depending on the deoxidizing conditions, but if the main phase issuch an oxide, they will sufficiently function as MnS precipitatingsites and the effect of increasing the fineness and hardness of theinclusions will not be impaired. To obtain such inclusions, the totalconcentration of the one or both of Ce or La must be made 0.0005% to0.04%. If the total concentration of the one or both of Ce or La is lessthan 0.0005%, the SiO₂ inclusions cannot be reduced, while if over0.04%, large amounts of cerium oxysulfide and lanthanum oxysulfide areproduced and form coarse inclusions which degrade the stretch flangeformability and fatigue characteristics.

Nb: 0.01 to 0.10%

Nb forms carbides, nitrides, and carbonitrides with C or N to promotethe increased fineness of the matrix structure. To obtain this effect,at least 0.01% is necessary. However, even if included in a large amountover 0.10%, the effect is saturated and the cost becomes high, so 0.10%is made the upper limit.

V: 0.01 to 0.05%

V forms carbides, nitrides, and carbonitrides with C or N to promote theincreased fineness of the matrix structure. To obtain this effect, atleast 0.01% is necessary. However, even if included in a large amountover 0.05%, the effect is saturated and the cost becomes high, so 0.05%is made the upper limit.

Cr: 0.01 to 0.6%

Cr may be included as necessary to improve the quenchability of steeland secure strength of the steel plate. To obtain this effect, at least0.01% is necessary. However, inclusion of a large amount converselydegrades the balance of strength and ductility. Therefore, 0.6% is madethe upper limit.

Mo: 0.01 to 0.4%

Mo may be included as necessary to improve the quenchability of steeland secure strength of the steel plate. To obtain this effect, at least0.01% is necessary. However, inclusion of a large amount converselydegrades the balance of strength and ductility. Therefore, 0.4% is madethe upper limit.

B: 0.0003 to 0.003%

B may be included as necessary to improve the quenchability of steel,strengthen the grain boundaries, and improve the workability. To obtainthis effect, at least 0.0003% is necessary. However, inclusion of alarge amount conversely detracts from the cleanliness of the steel anddegrades the ductility. Therefore, 0.003% is made the upper limit.

Next, the conditions of presence of the inclusions in the steel plate ofthe present invention will be explained. Further, the “steel plate”means the plate after rolling obtained by hot rolling or further coolingrolling.

To obtain the steel plate superior in stretch flange formability andfatigue characteristics, it is important to reduce as much as possiblethe stretched coarse MnS-based inclusions easily becoming startingpoints of cracking and routes for crack propagation in the steel plate.The inventors discovered through experiments that MnS-based inclusionswith a circle equivalent diameter of less than 1 μm are harmless asstarting points of cracking and do not cause deterioration of thestretch flange formability or fatigue characteristics. Further,inclusions with a circle equivalent diameter of 1 μm or more are easilyobserved by a scan type electron microscope (SEM) etc., so the inventorsinvestigated the shape and composition of inclusions in steel plate witha circle equivalent diameter of 1 μm or more and evaluated the state ofdistribution of the MnS-based inclusions. Here, the “circle equivalentdiameter” is defined as the (long axis×short axis)^(0.5) found from thelong axis and short axis of inclusions observed in cross-section.

Note that the upper limit of the circle equivalent diameter of theMnS-based inclusions is not particularly limited, but in practiceMnS-based inclusions of about 1 mm are observed.

The number ratio of the stretched inclusions is found by analyzing thecomposition of a plurality of randomly selected inclusions (for example50 or so) with a circle equivalent diameter of 1 μm or more using an SEMand measuring the long axes and short axes of the inclusions from theSEM image. Here, when defining “stretched inclusions” as inclusions witha long axis/short axis (stretch ratio) of 5 or more, the number ratio ofthe stretched inclusions can be found by dividing the detected number ofstretched inclusions by the total number of inclusions investigated (inthe above example, 50 or so).

Note that the stretch ratio of the inclusions was made 5 or more becausethe inclusions with a stretch ratio of 5 or more in comparative steelplate not containing La are almost all MnS-based inclusions. Further,the upper limit of the stretch ratio of the MnS-based inclusions is notparticularly limited, but in practice MnS-based inclusions with astretch ratio of 50 or so are sometimes observed.

As a result, it was learned that with steel plate controlled in form toa number ratio of stretched inclusions with a stretch ratio of 5 or moreof 20% or less, the stretch flange formability and the fatiguecharacteristics are improved. That is, if the number ratio of thestretched inclusions with a stretch ratio of 5 or more exceeds 20%, thenumber of MnS-based stretched inclusions easily becoming starting pointsof cracking becomes too large and the stretch flange formability and thefatigue characteristics drop. In the present invention, the number ratioof stretched inclusions with a stretch ratio of 5 or more is made 20% orless. Further, the stretch flange formability and the fatiguecharacteristics are better the small the number of stretched MnS-basedinclusions, so the lower limit value of the number ratio of thestretched inclusions with a stretch ratio of 5 or more includes 0%.

Here, the lower limit value of the number ratio of stretched inclusionswith a circle equivalent diameter of 1 μm or more and with a stretchratio of 5 or more being 0% means when there are inclusions with acircle equivalent diameter of 1 μm or more, but none with a stretchratio of 5 or more or when there are stretched inclusions with a stretchratio of 5 or more, but all have a circle equivalent diameter of lessthan 1 μm.

Further, in steel plate controlled to a form with a number ratio of thestretched inclusions with a stretch ratio of 5 or more of 20% or less,in accordance with this, MnS precipitates on the oxide or oxysulfide ofone or both of Ce or La. The form of the inclusions is not particularlylimited so long as MnS precipitates on an oxide or oxysulfide of one orboth of Ce or La, but usually is an oxide or oxysulfide of one or bothof Ce or La as core around which the MnS precipitates.

Further, inclusions comprised of an oxide or oxysulfide of one or bothof Ce or La on which MnS has precipitated are resistant to deformationeven at the time of rolling, so become unstretched shapes even in thesteel plate, that is, substantially spherical inclusions.

Here, the spherical inclusions judged as not stretched are notparticularly limited, but may be inclusions in the steel plate with astretch ratio of 3 or less, preferably inclusions with a ratio of 2 orless. This is because at the cast slab stage before the rolling, thestretch ratio of the inclusions of a form of an oxide or oxysulfide ofone or both of Ce or La on which MnS is precipitated was 3 or less.Further, if spherical inclusions judged as not stretched are completelyspherical, the stretch ratio would become 1, so the lower limit of thestretch ratio is 1.

The number ratio of the inclusions was investigated by a method similarto the investigation of the number ratio of the stretched inclusions. Asa result, it was learned that in steel plate controlled in precipitationto have a number ratio of inclusions of a form of an oxide or oxysulfideof one or both of Ce or La on which MnS is precipitated of 10% or more,the stretch flange formability and the fatigue characteristics areimproved. If the number ratio of inclusions of a form of an oxide oroxysulfide of one or both of Ce or La on which MnS is precipitatedbecomes less than 10%, in accordance with this, the number ratio ofMnS-based stretched inclusions becomes too large and the stretch flangeformability and the fatigue characteristics fall. For this reason, thenumber ratio of inclusions of a form of an oxide or oxysulfide of one orboth of Ce or La on which MnS is precipitated is made 10% or more.Further, the stretch flange formability and the fatigue characteristicsbecome better with a large amount of MnS precipitated on oxides oroxysulfides of one or both of Ce or La, so the upper limit value of thenumber ratio includes 100%.

Note that inclusions of a form of an oxide or oxysulfide of one or bothof Ce or La on which MnS is precipitated are resistant to deformationeven at the time of rolling, so the circle equivalent diameter is notparticularly limited, but may be 1 μm or more. However, if too large,the inclusions may form starting points of cracking, so the upper limitis preferably 50 μm or so.

On the other hand, not only are the inclusions resistant to deformationeven at the time of rolling, but when the circle equivalent diameter isless than 1 μm, they will also not form starting points of cracking, sothe lower limit of the circle equivalent diameter is not particularlydefined.

Next, as a condition of presence of inclusions in the steel plate of thepresent invention explained above, the number density of inclusions perunit volume is defined.

The distribution of particle size of the inclusions was obtained by SEMevaluation of the electrolyzed surface by the speed method. “SEMevaluation of the electrolyzed surface by the speed method” meanspolishing the surface of a sample piece, then electrolyzing it by thespeed method and directly evaluating the sample surface by an SEM toevaluate the size and number density of the inclusions. Note that the“speed method” is the method of using 10% acetyl acetone-1% tetramethylammonium chloride-methanol to electrolyze the sample surface and extractthe inclusions. As the amount of electrolysis, 1 C per 1 cm² area of thesample surface was electrolyzed. An SEM image of the thus electrolyzedsurface was processed to find the distribution of frequency (number)with respect to the circle equivalent diameter. From this distributionof frequency of the particle size, the average circle equivalentdiameter was calculated. Further, the frequency was divided by the depthfound by the area of the observed field and the amount of electrolysisto calculate the number density of inclusions per volume.

The inventors evaluated the volume number density of inclusions with acircle equivalent diameter of 1 μm or more and with a stretch ratio of 5or more becoming starting points of cracking and degrading the stretchflange formability and the fatigue characteristics and as a resultlearned that if 1.0×10⁴/mm³ or less, the stretch flange formability andthe fatigue characteristics are improved. If the volume number densityof stretched inclusions with a circle equivalent diameter of 1 μm ormore and with a stretch ratio of 5 or more is over 1.0×10⁴/mm³, thenumber density of the MnS-based stretched inclusions easily becomingstarting points of cracking becomes too large and the and the stretchflange formability and the fatigue characteristics fall, so the volumenumber density of stretched inclusions with a circle equivalent diameterof 1 μm or more and with a stretch ratio of 5 or more is made1.0×10⁴/mm³ or less. Further, the stretch flange formability and thefatigue characteristics are better the smaller the stretched MnS-basedinclusions, so the lower limit value of the volume number density with acircle equivalent diameter 1 μm or more and with a stretch ratio of 5 ormore includes 0%.

Here, the lower limit value of the volume number density of stretchedinclusions with a circle equivalent diameter of 1 μm or more and with astretch ratio of 5 or more being 0% means the same as the above.

Further, in steel plate controlled to a form with a volume numberdensity of stretched inclusions with a diameter of 1 μm or more and witha stretch ratio of 5 or more of 1.0×10⁴/mm³ or less, in accordance withthis, the unstretched MnS-based inclusions become a form of an oxide oroxysulfide of one or both of Ce or La on which MnS is precipitated. Theshape was substantially spherical inclusions.

The form of the inclusions, in the same way as the above, is notparticularly limited so long as it is an oxide or oxysulfide of one orboth of Ce or La on which MnS is precipitated, but usually it is anoxide or oxysulfide of one or both of Ce or La as a core around whichMnS is precipitated.

Further, the “spherical inclusions” is not particularly limited, butrefers to inclusions in the steel plate with a stretch ratio of 3 orless, preferably inclusions with a ratio of 2 or less. Here, ifcompletely spherical, the stretch ratio becomes 1, so the lower limit ofthe stretch ratio is 1.

The inventors investigated the volume number density of such inclusionsand as a result learned that with steel plate controlled inprecipitation to give a volume number density of inclusions of a form ofan oxide or oxysulfide of one or both of Ce or La as a core around whichMnS is precipitated of 1.0×10³/mm³ or more, the stretch flangeformability and the fatigue characteristics are improved. If the volumenumber density of inclusions of a form of an oxide or oxysulfide of oneor both of Ce or La on which MnS is precipitated is less than1.0×10³/mm³, in accordance with this, the number ratio of the MnS-basedstretched inclusions becomes too large and the stretch flangeformability and the fatigue characteristics fall, so the volume numberdensity of inclusions of a form of an oxide or oxysulfide of one or bothof Ce or La on which MnS is precipitated is defined as 1.0×10³/mm³ ormore. Further, the stretch flange formability and the fatigue strengthbecome better the more the MnS precipitated around cores of an oxide oroxysulfide of one or both of Ce or La, so the upper limit value of thevolume number density is not particularly defined.

Note that the circle equivalent diameter of inclusions of a form of anoxide or oxysulfide of one or both of Ce or La on which MnS isprecipitated, in the same way as above, is not particularly limited, butmay be 1 μm or more. However, if this circle equivalent diameter is toolarge, the inclusions are liable to become starting points of cracking,so the upper limit is preferably 50 μm or so.

On the other hand, when the circle equivalent diameter of the inclusionsis less than 1 μm, there is no problem at all, so the lower limit is notparticularly defined.

Next, as a condition of presence of stretched inclusions in the steelplate of the present invention described above, the upper limit of thecircle equivalent diameter is defined. Specifically, the inventorsevaluated the average circle equivalent diameter of inclusions with acircle equivalent diameter of 1 μm or more and with a stretch ratio of 5or more forming starting points of cracking and degrading the stretchflange formability and fatigue characteristics and as a result learnedthat if the average circle equivalent diameter of the stretchedinclusions is 10 μm or less, the stretch flange formability and fatiguecharacteristics are improved. The inventors took note of the fact thatalong with an increase in the number ratio of the stretched inclusionswith a circle equivalent diameter of 1 μm or more and with a stretchratio of 5 or more, the average circle equivalent diameter of thestretched inclusions becomes larger and defined the average circleequivalent diameter of the stretched inclusions as an indicator. Theyguessed that as the amount of Mn or S in the steel increases, the numberof MnS formed increases and the formed MnS becomes coarser in size.

Therefore, if the stretched inclusions with a circle equivalent diameterof 1 μm or more and with a stretch ratio of 5 or more exceed 10 μm, inaccordance with this, the number ratio of the stretched inclusionsexceeds 20%, so the number ratio of coarse MnS-based stretchedinclusions easily becoming starting points of cracking becomes too largeand the stretch flange formability and fatigue characteristics fall,therefore the average circle equivalent diameter of the stretchedinclusions with a circle equivalent diameter of 1 μm or more and with astretch ratio of 5 or more is made 10 μm or less.

Note that defining the average circle equivalent diameter of stretchedinclusions with a circle equivalent diameter of 1 μm or more and with astretch ratio of 5 or more as 10 μm or less means the case whereinclusions with a circle equivalent diameter of 1 μm or more are presentin the steel plate, so the lower limit value of the circle equivalentdiameter becomes 1 μm.

On the other hand, as a condition of presence of inclusions of a form ofan oxide or oxysulfide of one or both of Ce or La on which MnS isprecipitated in the steel plate of the present invention explainedabove, the content of the average composition of Ce or La in theinclusions where MnS is precipitated is defined.

Specifically, as explained above, in improving the stretch flangeformability and fatigue characteristics, it is important to make MnSprecipitate over an oxide or oxysulfide of one or both of Ce or La andprevent stretching of the MnS.

The form of the inclusions, in the same way as the above, is notparticularly limited so long as MnS precipitates on an oxide oroxysulfide of one or both of Ce or La, but in most cases it comprises anoxide or oxysulfide of one or both of Ce or La as a core around whichMnS is precipitated.

Further, the spherical inclusions are not particularly limited, but maybe inclusions in the steel plate with a stretch ratio of 3 or less,preferably inclusions with a ratio of 2 or less. Here, if completelyspherical, the stretch ratio is 1, so the lower limit of the stretchratio is 1.

Therefore, to clarify the composition effective for suppressingstretching of the MnS-based inclusions, the inventors analyzed thecomposition of inclusions of a form of an oxide or oxysulfide of one orboth of Ce or La on which MnS is precipitated.

However, if the circle equivalent diameter of the inclusions is 1 μm ormore, observation becomes easy, so for convenience they covered a circleequivalent diameter of 1 μm or more. However, if observation ispossible, inclusions with a circle equivalent diameter of less than 1 μmmay also be included.

Further, inclusions of a form of an oxide or oxysulfide of one or bothof Ce or La on which MnS is precipitated do not stretch, so it wasconfirmed that the stretch ratio was 3 or less in all of the inclusions.Therefore, the inventors analyzed the composition of inclusions with acircle equivalent diameter of 1 μm or more and with a stretch ratio of 3or less.

As a result, they learned that if inclusions with a circle equivalentdiameter of 1 μm or more and with a stretch ratio of 3 or less contain,in average composition, a total of one or both of Ce or La of 0.5 to50%, the stretch flange formability and the fatigue characteristics areimproved. If the average content of the total of one or both of Ce or Lain the inclusions with a circle equivalent diameter of 1 μm or more anda stretch ratio of 3 or less becomes less than 0.5 mass %, the numberratio of the inclusions of a form of an oxide or oxysulfide of one orboth of Ce or La on which MnS is precipitated is greatly reduced and, inaccordance with this, the number ratio of MnS-based stretched inclusionseasily becoming starting points of cracking becomes too large and thestretch flange formability and fatigue characteristics fall.

On the other hand, if the average content of the total of one or both ofCe or La in the inclusions with a circle equivalent diameter of 1 μm ormore and with a stretch ratio of 3 or less exceeds 50%, large amounts ofcerium oxysulfides and lanthanum oxysulfides are formed and coarseinclusions with a circle equivalent diameter of 50 μm or so or more areformed, so the stretch flange formability and fatigue characteristicsare degraded.

Further, as a condition of presence of inclusions of a form of an oxideor oxysulfide of one or both of Ce or La on which MnS is precipitated inthe steel plate of the present invention, the chemical ingredient(Ce+La)/S ratio of the steel plate is defined.

Specifically, as explained above, in improving the stretch flangeformability and fatigue characteristics, the ratio of chemicalingredients for making MnS precipitate on an oxide or oxysulfide of oneor both of Ce or La and preventing stretching of the MnS is important.

Therefore, to clarify the ratio of chemical ingredients effective forsuppressing stretching of MnS-based inclusions, the inventors changedthe (Ce+La)/S ratio of the steel plate and evaluated the form of theinclusions, stretch flange formability, and fatigue characteristics(FIG. 1). As a result, they learned that when the (Ce+La)/S ratio is 0.1to 70, the stretch flange formability and the fatigue characteristicsare improved. If the (Ce+La)/S ratio becomes less than 0.1, the numberratio of inclusions of a form of an oxide or oxysulfide of one or bothof Ce or La on which MnS is precipitated is greatly reduced, and, inaccordance with this, the number ratio of MnS-based stretched inclusionseasily becoming starting points of cracking becomes too large and thestretch flange formability and fatigue characteristics fall.

On the other hand, if the (Ce+La)/S ratio exceeds 70, cerium oxysulfidesand lanthanum oxysulfides are formed in large amounts and form coarseinclusions with a circle equivalent diameter of 50 μm or so or more, sothe stretch flange formability and the fatigue characteristics aredegraded.

Next, the structure of the steel plate will be explained.

The present invention improves the stretch flange formability andfatigue characteristics by control of the MnS-based inclusions. Themicrostructure of the steel plate is not particularly limited. Theeffect of the present invention is obtained in any steel plate of steelplate of a structure with bainitic ferrite as a main phase, compositestructure steel plate having a ferrite phase as a main phase and havinga martensite phase or bainite phase as a second phase, and compositestructure steel plate comprised of ferrite, residual austenite, and alow temperature transformed phase (martensite or bainite), but to obtaina superior stretch flange formability, making the structure one havingbainitic ferrite as its main phase is preferred. Preferably the bainiticferrite or bainite phase is the largest phase in terms of area ratio.The area rate of the bainitic ferrite phase is preferably 50% or more,more preferably 80% or more, still more preferably 100%. Further, thebalance may be made a bainite phase or polygonal ferrite phase containedin an amount of 20% or more.

Next, the production conditions will be explained. In the presentinvention, the molten steel is blow refined in a converter todecarburize it and is further decarburized by using a vacuum degassingapparatus to make the C concentration 0.03 to 0.1%. Si, Mn, P, and otheralloys are added to this molten steel for deoxidation and adjustment ofthe ingredients. Along with this either Al and Ti are not added or, whenadjustment of the oxygen is necessary, a small amount of Al or Ti of anextent whereby a small amount of acid soluble Al or acid soluble Tiremains is added, then one or both of Ce or La is added to adjust thecomposition. The thus produced molten steel is continuously cast toproduce a cast slab.

Regarding the continuous casting, not only may the invention be appliedto continuous casting of slabs of an extent of the usually 250 mmthickness, but it may also be sufficiently applied to continuous castingof blooms or billets or of thin slabs produced by slab continuouscasting machine with thicknesses of the casting molds thinner thanusual, for example, 150 mm or less.

The hot rolling conditions for producing high strength hot rolled steelplate will be explained next. The heating temperature of the slabsbefore hot rolling is preferably 1150° C. or more for making thecarbonitrides etc. in the steel enter solid solution. By making theseenter solid solution, the formation of polygonal ferrite is suppressedin the cooling process after rolling and a structure mainly comprised ofa bainitic ferrite phase preferable for the stretch flange formabilityis obtained. On the other hand, if the heating temperature of the slabbefore hot rolling exceeds 1250° C., the oxidation of the slab surfacebecomes remarkable. In particular, the grain boundaries are selectivelyoxidized. Due to this, wedge-shaped surface defects remain afterdescaling. This detracts from the surface quality after rolling, so theupper limit is preferably made 1250° C.

After heating to the above temperature range, the usual hot rolling isperformed, but during this process, the finish rolling end temperatureis important when controlling the structure of the steel plate. When thefinish rolling end temperature is less than the Ar₃ point+30° C., thecrystal grains at the surface layer easily become coarser. This is notpreferable for the fatigue characteristics. On the other hand, if overthe Ar₃ point+200° C., a polygonal ferrite phase not preferable for thestretch flange formability is easily formed, so the upper limit ispreferably made the Ar₃ point+200° C.

Further, making the average cooling rate of the steel plate after finishrolling 40° C./s or more and cooling in the range up to 300 to 500° C.is effective for suppressing the formation of the polygonal ferritephase and obtaining a structure mainly comprised of a bainitic ferritephase.

If the average cooling rate is less than 40° C./s, polygonal ferritephase forms more easily, so this is not preferred. On the other hand,for control of the structure, it is not necessary to provide an upperlimit for the cooling rate, but too fast a cooling rate is liable tomake the cooling of the steel plate uneven. Further, construction of afacility enabling such cooling requires tremendous costs. This isbelieved to invite a rise in the price of steel plate. From such aviewpoint, the upper limit of the cooling rate is preferably made 100°C./s.

Further, if the cooling stop temperature becomes lower than 300° C., amartensite phase not preferable for stretch flange formability isformed, so the lower limit was made 300° C. Therefore, the coilingtemperature of the hot rolled coil is preferably made 300° C. or morefor suppressing the formation of a martensite phase causing extremedeterioration of the stretch flange formability.

On the other hand, if over 500° C., formation of a polygonal ferritephase cannot be suppressed. Further, in steel containing Cu, Cu isliable to locally precipitate in the ferrite phase and lower the effectof improvement of the fatigue characteristics, so the coilingtemperature is preferably made 500° C. or less. Therefore, by coiling at500° C. or less, carbonitrides are precipitated in the subsequentcooling process so reduce the amounts of solid solution C and N in theferrite phase and cause an improvement in the stretch flangeformability.

EXAMPLES

Below, examples of the present invention will be explained along withcomparative examples.

Slabs having the chemical ingredients shown in Table 1 were hot rolledunder the conditions shown in Table 2 to obtain hot rolled plates of athickness of 3.2 mm.

TABLE 1 Acid Acid Steel sol. sol. No. C Si Mn P S N Al Ti Cr Inv. Ex. 11 0.07 0.20 1.3 0.015 0.0050 0.0025 0.006 Comp. Ex. 1 2 0.07 0.19 1.30.015 0.0050 0.0026 0.035 Inv. Ex. 2 3 0.065 0.18 1.5 0.012 0.01000.0022 0.004 0.005 Comp. Ex. 2 4 0.065 0.18 1.5 0.012 0.0100 0.00230.040 0.005 Inv. Ex. 3 5 0.095 1.00 2.8 0.010 0.0080 0.0030 0.002 0.002Comp. Ex. 3 6 0.095 1.00 2.8 0.009 0.0080 0.0028 0.038 0.002 Inv. Ex. 47 0.035 1.00 1.4 0.010 0.0200 0.0020 0.003 Comp. Ex. 4 8 0.035 1.00 1.390.010 0.0200 0.0021 0.003 Inv. Ex. 5 9 0.06 0.68 1.38 0.010 0.00400.0020 0.003 0.002 Comp. Ex. 5 10 0.06 0.69 1.38 0.010 0.0040 0.00210.003 0.002 Inv. Ex. 6 11 0.06 0.68 1.38 0.010 0.0007 0.0020 0.003 0.002Comp. Ex. 6 12 0.06 0.69 1.38 0.010 0.0007 0.0021 0.003 0.002 Inv. Ex. 713 0.1 0.25 2 0.010 0.0030 0.0020 0.003 0.002 0.03 Comp. Ex. 7 14 0.10.25 2 0.010 0.0030 0.0021 0.003 0.002 0.03 Nb V Mo B Cu Ni Ce La (Ce +La)/S Inv. Ex. 1 0.0010 0.2 Comp. Ex. 1 Inv. Ex. 2 0.02 0.1 0.05 0.00500.0030 0.8 Comp. Ex. 2 0.02 0.1 0.05 Inv. Ex. 3 0.0200 2.5 Comp. Ex. 3Inv. Ex. 4 0.10 0.0250 1.25 Comp. Ex. 4 0.10 0.0003 0.0015 Inv. Ex. 50.0070 1.75 Comp. Ex. 5 0.0003 0.075 Inv. Ex. 6 0.0050 7.143 Comp. Ex. 6Inv. Ex. 7 0.03 0.02 0.15 0.002 0.0050 0.0030 2.667 Comp. Ex. 7 0.030.02 0.15 0.002 0.0002 0.067

TABLE 2 Finish Cooling rate Heating rolling end after finish CoilingCondi- temperature temperature rolling temperature tions (° C.) (° C.)(° C./s) (° C.) A 1250 845 75 450 B 1200 825 45 450

In this Table 1, Steel Numbers (hereinafter referred to as “Steel Nos.”)1, 3, 5, 7, 9, 11, and 13 are made compositions in the range of highstrength steel plate according to the present invention, while SteelNos. 2, 4, 6, 8, 10, 12, and 14 are made comparative steels outside therange of high strength steel plate according to the present invention.Steel Nos. 2, 4, and 6 were made slabs containing acid soluble Al inover 0.01%, while Steel Nos. 8, 10, 12, and 14 were made slabs with thetotal of one or both of Ce or La reduced to less than 0.0005.

In this regard, in Table 1, to enable the Steel No. 1 and the Steel No.2, the Steel No. 3 and the Steel No. 4, the Steel No. 5 and the SteelNo. 6, and the Steel No. 7 and the Steel No. 8 to be compared, they weremade substantially the same in composition and made different in acidsoluble Al etc. Further, to enable the Steel No. 9 and Steel No. 10, theSteel No. 11 and Steel No. 12, and the Steel No. 13 and Steel No. 14 tobe compared, they were made substantially the same in composition andmade different in Ce+La etc.

Further, in this Table 2, the Conditions A were made a heatingtemperature of 1250° C., a finish rolling end temperature of 845° C., acooling rate after finish rolling of 75° C./s, and a coiling temperatureof 450° C., while the Conditions B were made a heating temperature of1200° C., a finish rolling end temperature of 825° C., a cooling rateafter finish rolling of 45° C./s, and a coiling temperature of 450° C.

For the Steel No. 1 and the Steel No. 2, the Conditions A were applied,further for the Steel No. 3 and the Steel No. 4, the Conditions B wereapplied, for the Steel No. 5 and the Steel No. 6, the Conditions A wereapplied, and for the Steel No. 7 and the Steel No. 8, the Steel No. 9and the Steel No. 10, the Steel No. 11 and the Steel No. 12, and theSteel No. 13 and the Steel No. 14, the Conditions B were applied toenable the effects of the chemical compositions to be compared under thesame production conditions.

As basic characteristics of the steel plates obtained in this way, theinventors investigated the strength, ductility, stretch flangeformability, and fatigue strength ratio.

Further, as the state of presence of stretched inclusions in the steelplate, the inventors investigated the number ratio, volume numberdensity, and average circle equivalent diameter of inclusions having astretch ratio of 5 or more for all inclusions of 1 μm or more.

Furthermore, as the state of presence of unstretched inclusions in thesteel plate, the inventors investigated the number ratio and volumenumber density of inclusions comprised of an oxide or oxysulfide of oneor both of Ce or La on which MnS has precipitated for all inclusions of1 μm or more and the average value of contents of the total of one orboth of Ce or La in the inclusions with a stretch ratio of 3 or less.

Note that inclusions of 1 μm or more were covered because of the ease ofobservation and, in addition, the fact that inclusions of less than 1 μmdo not have any effect on deterioration of the stretch flangeformability and fatigue characteristics.

The results are shown in Table 3 for each combination of steel androlling conditions.

TABLE 3 Average circle Average content equivalent of total of onediameter in or both of Ce or inclusions with La in inclusions circlewith circle equivalent equivalent diameter 1 μm or Ce + Ce + MnSStretched diameter 1 μm or more and MnS Stretched volume volume more andstretch stretch ratio 5 Hole Fatigue number number number number ratio 3or less or more expansion strength Condition Strength Ductility ratioratio density density (%) (μm) value ratio A 460 23 90 0 2.5 × 10⁴ 0 2 6160 0.67 A 458 20 0 75 0 2.8 × 10⁴ 0 12 110 0.48 B 497 22 80 5 6.1 × 10⁴4.1 × 10³ 10 7 162 0.6 B 495 19 0 85 0 7.0 × 10⁴ 0 22 105 0.46 A 1150 1185 0 2.8 × 10⁴ 0 30 5 80 0.64 A 1140 8 0 80 0 4.7 × 10⁴ 0 18 45 0.45 B810 22 87 7 9.6 × 10⁴ 7.2 × 10³ 33 9 100 0.61 B 805 20 1 97 9.6 × 10²9.3 × 10⁴ 0.4 24 62 0.44 B 605 25 83 6 8.8 × 10⁴ 6.1 × 10³ 29 8 90 0.68B 605 25 1 95 8.4 × 10² 9.3 × 10⁴ 0.4 23 38 0.49 B 605 25 87 0 9.5 × 10⁴0 50 5 100 0.68 B 605 25 0 97 0 9.4 × 10⁴ 0 23 34 0.49 B 1005 17 86 59.7 × 10⁴ 5.9 × 10³ 34 7 75 0.63 B 995 16 1 96 8.2 × 10² 9.4 × 10⁴ 0.324 30 0.44

The strength and ductility were found by a tensile test of a JIS No. 5test piece taken in parallel with the rolling direction. The stretchflange formability was evaluated by pushing open a punched hole of adiameter of 10 mm made at the center of a 150 mm×150 mm steel plate by a60° conical punch, measuring the hole diameter D (mm) when a crackoccurs passing through the plate thickness, and finding the holeexpansion value λ=(D−10)/10. Further, the fatigue strength ratio used asan indicator showing the fatigue characteristics was evaluated by thevalue of the fatigue strength at 2×10⁶ cycles (σW) found by the methodbased on JIS Z 2275 divided by the strength (σB) of the steel plate(σW/σB).

Note that the test piece used was a No. 1 test piece defined in thespecification having a parallel part of 25 mm, a radius of curvature Rof 100 mm, and a thickness after equally grinding the two surfaces ofthe original plate (hot rolled plate) of 3.0 mm.

Furthermore, the inclusions were observed under a SEM. Fifty randomlyselected inclusions with a circle equivalent diameter of 1 μm or morewere measured for their long axes and short axes. Furthermore, thequantitative analysis function of an SEM was used to analyze thecomposition of 50 randomly selected inclusions with a circle equivalentdiameter of 1 μm or more. Using these results, the number ratio ofinclusions with a stretch ratio of 5 or more, the average circleequivalent diameter of inclusions with a stretch ratio of 5 or more, thenumber ratio of inclusions comprised of an oxide or oxysulfide of one orboth of Ce or La on which MnS has precipitated, and furthermore anaverage value of the total of one or both of Ce or La in the inclusionswith a stretch ratio of 3 or less were found. Further, the volume numberdensity by type of inclusions was calculated by evaluation of theelectrolyzed surface by SEM evaluation by the speed method.

As clear from Table 3, in Steel Nos. 1, 3, 5, 7, 9, 11, and 13 applyingthe method of the present invention, by making MnS precipitate at anoxide or oxysulfide of one or both of Ce or La, it was possible toreduce the stretched MnS-based inclusions in the steel plate. That is,by making the number ratio of inclusions comprised of an oxide oroxysulfide of one or both of Ce or La on which MnS is precipitated inthe steel plate 10% or more, making the volume number density of theinclusions 1.0×10³/mm³ or more, and making the average content of thetotal of one or both of Ce or La in the inclusions with a stretch ratioof 3 or less present in the steel plate 0.5% to 50%, it was possible tomake the number ratio of the stretched inclusions with a circleequivalent diameter 1 μm or more and with a stretch ratio of 5 or more20% or less, make the volume number density of the inclusions1.0×10⁴/mm³ or less, and make the average circle equivalent diameter ofthe inclusions 10 μm or less. As a result, compared with the comparativesteels, in the invention steels of Steel Nos. 1, 3, 5, 7, 9, 11, and 13,steel plate superior in stretch flange formability and fatiguecharacteristics could be obtained. However, in the comparative steels(Steel Nos. 2, 4, 6, 8, 10, 12, and 14), the state of distribution ofthe stretched MnS-based inclusions and inclusions comprised of an oxideor oxysulfide of one or both of Ce or La at which MnS has beenprecipitated differs from the state of distribution prescribed in thepresent invention, so at the time of working the steel plate, thestretched MnS-based inclusions formed starting points of cracking andthe stretch flange formability and the fatigue characteristics dropped.

INDUSTRIAL APPLICABILITY

According to the method of the present invention, by making fine MnSprecipitate in the slab and making them disperse in the steel plate asfine spherical inclusions not being deformed at the time of rolling andnot easily forming starting points of cracking, high strength hot rolledsteel plate superior in stretch flange formability and fatiguecharacteristics can be obtained.

1. A high strength steel plate superior in stretch flange formabilityand fatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having a number ratio of stretchedinclusions present in the steel plate having a circle equivalentdiameter of 1 μm or more and a long axis/short axis of 5 or more of 20%or less.
 2. A high strength steel plate superior in stretch flangeformability and fatigue characteristics characterized by comprisingsteel plate containing, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%,Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: less than0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%, havinga balance of iron and unavoidable impurities, and having inclusions inthe steel plate comprised of an oxide or oxysulfide of one or both of Ceor La on which MnS is precipitated in a number ratio of 10% or more. 3.A high strength steel plate superior in stretch flange formability andfatigue characteristics characterized by comprising steel platecontaining, by mass %, C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acidsoluble Al: 0.01% or less, acid soluble Ti: less than 0.008%, and atotal of one or both of Ce or La: 0.0005 to 0.04%, having a balance ofiron and unavoidable impurities, and having a volume number ratio ofstretched inclusions present in the steel plate having a circleequivalent diameter of 1 μm or more and a long axis/short axis of 5 ormore of 1.0×10⁴/mm³ or less.
 4. A high strength steel plate superior instretch flange formability and fatigue characteristics characterized bycomprising steel plate containing, by mass %, C: 0.03 to 0.20%, Si: 0.08to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N:0.0005 to 0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: lessthan 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%,having a balance of iron and unavoidable impurities, and having a volumenumber density of inclusions in the steel plate comprised of an oxide oroxysulfide of one or both of Ce or La on which MnS is precipitated in avolume number density of 1.0×10³/mm³ or more.
 5. A high strength steelplate superior in stretch flange formability and fatigue characteristicscharacterized by comprising steel plate containing, by mass %, C: 0.03to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S:0.0005% or more, N: 0.0005 to 0.01%, acid soluble Al: 0.01% or less,acid soluble Ti: less than 0.008%, and a total of one or both of Ce orLa: 0.0005 to 0.04%, having a balance of iron and unavoidableimpurities, and having an average circle equivalent diameter ofstretched inclusions present in the steel plate having a circleequivalent diameter of 1 μm or more and a long axis/short axis of 5 ormore of 10 μm or less.
 6. A high strength steel plate superior instretch flange formability and fatigue characteristics characterized bycomprising steel plate containing, by mass %, C: 0.03 to 0.20%, Si: 0.08to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N:0.0005 to 0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: lessthan 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%,having a balance of iron and unavoidable impurities, having inclusionspresent in the steel plate comprising an oxide or oxysulfide of one orboth of Ce or La on which MnS is precipitated, and having the inclusionsinclude, in average composition, a total of one or both of Ce or La in0.5 to 50 mass %.
 7. A high strength steel plate superior in stretchflange formability and fatigue characteristics characterized bycomprising steel plate containing, by mass %, C: 0.03 to 0.20%, Si: 0.08to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N:0.0005 to 0.01%, acid soluble Al: 0.01% or less, acid soluble Ti: lessthan 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%,having a balance of iron and unavoidable impurities, and having a(Ce+La)/S ratio of 0.1 to
 70. 8. A high strength steel plate superior instretch flange formability and fatigue characteristics as set forth inany one of claims 1 to 7 characterized by comprising steel platecontaining, by mass %, one or more of any of Nb: 0.01 to 0.10%, V: 0.01to 0.05%, Cr: 0.01 to 0.6%, Mo: 0.01 to 0.4%, and B: 0.0003 to 0.03% andhaving a balance of iron and unavoidable impurities.